Basically, a television picture tube consists of an electron emitting means and a surrounding envelope comprising a funnel portion and a viewing faceplate. The faceplate is fabricated from a transparent glass having a composition which is highly absorptive of X-radiation and strongly resistant to discoloration resulting from the impingement of X-radiation and high velocity electrons thereupon.
U.S. Pat. No. 2,477,329 taught the inclusion of CeO.sub.2 in glass compositions designed for use as television picture tube faceplates to inhibit discoloration of "browning" caused by X-radiation and the essential exclusion of readily-reducible oxides, specifically noting PbO, to minimize discoloration resulting from electron bombardment. The patent employed BaO as the primary X-radiation absorbing component.
U.S. Pat. No. 3,464,932 disclosed the utility of alkali metal silicate glasses containing SrO as the principal X-ray absorbing ingredient for the manufacture of television tube faceplates and, since the issuance of that patent, compositions containing substantial quantities of SrO have comprised the commercial standard for color television picture tube faceplates produced in the United States. The patent described glasses consisting essentially, in weight percent, of 4-20% Na.sub.2 O+K.sub.2 O consisting of 0-10% Na.sub.2 O and 0-10% K.sub.2 O, an effective amount up to 20% SrO, and 40-70% SiO.sub.2, the sum of Na.sub.2 O+K.sub.2 O+SrO+SiO.sub.2 constituting at least 70% of the total composition. The patent discloses the optional inclusions of up to 20% BaO, up to 15% total of CaO, MgO, and ZnO, up to 10% Al.sub.2 O.sub.3, up to 10% ZrO.sub.2, up to 2% F, up to 5% B.sub.2 O.sub.3, up to 5% P.sub.2 O.sub.5, and, while desirably absent, up to 3% PbO. Thus, excessive PbO can result in electron browning.
U.S. Pat. No. 3,805,107 was also directed to SrO-containing glasses suitable for use as television picture tube faceplates. In the commercial manufacture of such faceplates, fluorine had been included in the batch materials as a fluxing agent. Because of its volatilization during melting of the batch, however, air pollution in the vicinity of the melting unit became a recognized environmental problem. Accordingly, the patent delineated ranges of components which would yield glasses having melting and forming properties satisfactory for the production of faceplates while retaining the necessary physical characteristics required for that application. The patent also noted the complementary effect of TiO.sub.2 with CeO.sub.2 in inhibiting X-ray browning, disclosing the utility of up to 2% for that purpose. The glass compositions described consisted essentially, in weight percent of 5-20% SrO, 0-15% BaO, 5-20% SrO+BaO, 4-13% K.sub.2 O, 4-13% Na.sub.2 O, 8-20% K.sub.2 O+Na.sub.2 O, 0.2-2% Li.sub.2 O, 1.5-4% Al.sub.2 O.sub.3, 0-5% CaO, 0- 5% MgO, 0.25-2% TiO.sub.2, 0.05-0.5% CeO.sub.2, and the balance SiO.sub.2.
U.S. Pat. No. 3,819,972 was drawn to SrO-containing glasses operable as faceplates for television picture tubes which statedly demonstrated superior working properties because the annealing points thereof were at least 550.degree. C. Such high annealing points purportedly aided in the sealing operation when the faceplate was sealed to the funnel portion of the picture tube. The crux of the invention consisted in employing glasses containing more than 14% K.sub.2 O and no more than 1.6% Na.sub.2 O. The compositions disclosed consisted essentially, in weight percent, of 45-70% SiO.sub.2, 14-25% K.sub.2 O, 0-1.6% Na.sub.2 O, 0-6% Al.sub.2 O.sub.3, 0-8% CaO, 0-3% MgO, 0-20% BaO, up to 20% SrO, 0-30% PbO, 0-2% Li.sub.2 O, 0-2% F.sub.2, 0-2% MnO, 0-1% TiO.sub.2, 0-1% Sb.sub.2 O.sub.3, 0-1% CeO.sub.2, and 0-1% rare earth oxides.
U.S. Pat. No. 3,925,089 described the use of certain feldspars and quartz sands as batch ingredients for making the funnel and faceplate portions of television picture tubes. The glass compositions prepared from the batches consisted essentially, in weight percent, of 50-75% SiO.sub.2, 0.5-6% Al.sub.2 O.sub.3, 0-5% CaO+MgO, Li.sub.2 O+Na.sub.2 O+K.sub.2 O&gt;13%, 0-1% B.sub.2 O.sub.3, 0-15% PbO, 0-15% BaO, 0-8% ZrO.sub.2, 0-12% SrO, PbO+BaO+ZrO.sub.2 +SrO&gt;8%.
U.S. Pat. No. 4,015,966 taught the production of SrO-containing glasses suitable for use as television picture tube faceplates which can be drawn as sheet on a bath of molten tin in accordance with the float glass process. The compositions disclosed consisted essentially, in weight percent, of 60-65% SiO.sub.2, 0-5% Al.sub.2 O.sub.3, 2-10% Na.sub.2 O, 0-17% K.sub.2 O, 2-10% CaO+MgO, 0-5% BaO, 5-15% SrO, 0-10% ZrO.sub.2, 0-5% WO.sub.3, 0-1% TiO.sub.2, 0-1% CeO.sub.2, 0.1-1% TiO.sub.2 +CeO.sub.2, and 0-1% Fe.sub.2 O.sub.3. The nub of the invention is stated to be the absence of PbO, Sb.sub.2 O.sub.3, and As.sub.2 O.sub.3 from the glass compositions, thereby eliminating reduction thereof in the non-oxidizing atmosphere demanded in the float glass process.
U.S. Pat. No. 4,065,697 described the preparation of SrO-containing glasses for use in television picture tubes. Operable glasses consisted essentially, in weight percent, of 45-70% SiO.sub.2, 3-10% Na.sub.2 O, 5-15% K.sub.2 O, 0-6% Al.sub.2 O.sub.3, 0-8% CaO, 0-3% MgO, 0-20% BaO, up to 15% SrO, 0-30% PbO, 0-2% Li.sub.2 O, 0-2% F.sub.2, 0-2% MnO, 0-1% TiO.sub.2, 0-1% Sb.sub.2 O.sub.3, 0-1% CeO.sub.2, and 0-1% rare earths. The preferred compositions consisted essentially, in weight percent, of 50-70% SiO.sub.2, 3-10% Na.sub.2 O, 5-15% K.sub.2 O, up to 15% SrO, 3-5% Al.sub.2 O.sub.3, 1-5% CaO, 0.1-20% BaO, 0.1-3% MgO, 0-2% F.sub.2, and 0.1-5% PbO.
One problem that has plagued the melting and forming capabilities of SrO-containing glasses utilized in the production of television picture tube faceplates has been a relative high liquidus temperature, viz., greater than 800.degree. C. and, frequently, in excess of 850.degree. C. This circumstance has led to undesirably low liquidus viscosities from the standpoint of ease in forming glass shapes. Laboratory experimentation in seeking means for reducing the liquidus temperature has commonly resulted in substantial changes in other physical properties of the glass, in particular a reduction in the softening, annealing and strain points of the glass and/or a substantial variation in the coefficient of thermal expansion thereof.